1 /*
2 * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.util;
27
28 import java.util.function.Consumer;
29
30 /**
31 * An iterator over a collection. {@code Iterator} takes the place of
32 * {@link Enumeration} in the Java Collections Framework. Iterators
33 * differ from enumerations in two ways:
34 *
35 * <ul>
36 * <li> Iterators allow the caller to remove elements from the
37 * underlying collection during the iteration with well-defined
38 * semantics.
39 * <li> Method names have been improved.
40 * </ul>
41 *
42 * <p>This interface is a member of the
43 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
44 * Java Collections Framework</a>.
45 *
46 * @apiNote
47 * An {@link Enumeration} can be converted into an {@code Iterator} by
48 * using the {@link Enumeration#asIterator} method.
49 *
50 * @param <E> the type of elements returned by this iterator
51 *
52 * @author Josh Bloch
53 * @see Collection
54 * @see ListIterator
55 * @see Iterable
56 * @since 1.2
57 */
58 public interface Iterator<E> {
59 /**
60 * Returns {@code true} if the iteration has more elements.
61 * (In other words, returns {@code true} if {@link #next} would
62 * return an element rather than throwing an exception.)
63 *
64 * @return {@code true} if the iteration has more elements
65 */
66 boolean hasNext();
67
68 /**
69 * Returns the next element in the iteration.
70 *
71 * @return the next element in the iteration
72 * @throws NoSuchElementException if the iteration has no more elements
73 */
74 E next();
75
76 /**
77 * Removes from the underlying collection the last element returned
78 * by this iterator (optional operation). This method can be called
79 * only once per call to {@link #next}.
80 * <p>
81 * The behavior of an iterator is unspecified if the underlying collection
82 * is modified while the iteration is in progress in any way other than by
83 * calling this method, unless an overriding class has specified a
84 * concurrent modification policy.
85 * <p>
86 * The behavior of an iterator is unspecified if this method is called
87 * after a call to the {@link #forEachRemaining forEachRemaining} method.
88 *
89 * @implSpec
90 * The default implementation throws an instance of
91 * {@link UnsupportedOperationException} and performs no other action.
92 *
93 * @throws UnsupportedOperationException if the {@code remove}
94 * operation is not supported by this iterator
95 *
96 * @throws IllegalStateException if the {@code next} method has not
97 * yet been called, or the {@code remove} method has already
98 * been called after the last call to the {@code next}
99 * method
100 */
101 default void remove() {
102 throw new UnsupportedOperationException("remove");
103 }
104
105 /**
106 * Performs the given action for each remaining element until all elements
107 * have been processed or the action throws an exception. Actions are
108 * performed in the order of iteration, if that order is specified.
109 * Exceptions thrown by the action are relayed to the caller.
110 * <p>
111 * The behavior of an iterator is unspecified if the action modifies the
112 * collection in any way (even by calling the {@link #remove remove} method
113 * or other mutator methods of {@code Iterator} subtypes),
114 * unless an overriding class has specified a concurrent modification policy.
115 * <p>
116 * Subsequent behavior of an iterator is unspecified if the action throws an
117 * exception.
118 *
119 * @implSpec
120 * <p>The default implementation behaves as if:
121 * <pre>{@code
122 * while (hasNext())
123 * action.accept(next());
124 * }</pre>
125 *
126 * @param action The action to be performed for each element
127 * @throws NullPointerException if the specified action is null
128 * @since 1.8
129 */
130 default void forEachRemaining(Consumer<? super E> action) {
131 Objects.requireNonNull(action);
132 while (hasNext())
133 action.accept(next());
134 }
135 }